Data stored in various types of data storage devices such as magnetic or optical disc drives, probe storage devices, or solid-state devices, could be exposed to periodic inspections by an unauthorized party or adversary.
One way to protect data on such devices is to encrypt the data using, for example, a public key or private key encryption scheme. An adversary not knowing the key, cannot decrypt the data, but can make snapshots of the encrypted data. By comparing different snapshots, an adversary could gain some information about the stored data. The analysis of changes in data over time is referred to as traffic analysis. For example, changes in the file allocation table (or another file system area) can reveal the extent of data changes, but most importantly, changes in a database may reveal information about the changed records.
An example would be a flight reservation system in which the adversary makes a reservation and later, having gained access to the storage medium, finds where changes were made in the reservation system database. By knowing where the database changed in response to the reservation, the adversary can gain knowledge about the data structure, the locations of the records for specific flights, seats, airlines, etc. With this knowledge, targeted reservations could be followed and overbooking provoked. In this manner, an adversary may cause significant damage to an airline that relies on the reservation system database.